1. Field of the Invention
The present invention is directed generally to wireless communication systems and, more specifically, to the transmission of acknowledgement signals in an UpLink (UL) of a communication system using time division multiplexing (TDM).
2. Description of the Art
A communication system includes a DownLink (DL), conveying transmissions of signals from a Base Station (BS or NodeB) to User Equipments (UEs), and the UL, conveying transmissions of signals from UEs to the NodeB. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile, such as a wireless device, a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other similar terminology.
The UL supports transmissions of data signals carrying information content, control signals providing information associated with the transmission of data signals in the DL, and Reference Signals (RSs), which are also commonly referred to as pilot signals. The DL also supports transmissions of data signals, control signals, and RSs.
UL data signals are conveyed through a Physical Uplink Shared CHannel (PUSCH). DL data channels are conveyed through a Physical Downlink Shared CHannel (PDSCH). In the absence of PUSCH transmission, a UE conveys Uplink Control Information (UCI) through a Physical Uplink Control CHannel (PUCCH). In the presence of PUSCH transmission, a UE may convey UCI together with data information through the PUSCH.
DL control signals may be of broadcast or UE-specific nature. UE-specific control signals can be used, for example, to provide Scheduling Assignments (SAs) to a UE for PDSCH reception (DL SAs) or PUSCH transmission (UL SAs). The NodeB transmits an SA using a Physical Downlink Control CHannel (PDCCH).
UL control signals include ACKnowledgement signals associated with a Hybrid Automatic Repeat reQuest (HARD) process (HARQ-ACK signals) and are typically transmitted in response to PDSCH receptions.
FIG. 1 illustrates a conventional PUCCH structure for HARQ-ACK signal transmission in a Transmission Time Interval (TTI), which consists of one sub-frame.
Referring to FIG. 1, a sub-frame 110 includes two slots 120. Each slot 120 includes NsymbUL, symbols for transmitting HARQ-ACK signals 130 and RSs 140, which enable coherent demodulation of the HARQ-ACK signals. Each symbol further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The transmission in the first slot may be at a different part of an operating BandWidth (BW) than in the second slot in order to provide frequency diversity. The operating BW includes frequency resource units that are referred to as Physical Resource Blocks (PRBs). Each PRB includes NscRB sub-carriers, or Resource Elements (REs), and a UE transmits HARQ-ACK signals and RSs over one PRB 150.
FIG. 2 illustrates the HARQ-ACK signal transmission in a sub-frame slot for the PUCCH structure in FIG. 1.
Referring to FIG. 2, b HARQ-ACK bits 210 modulate a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence 230 in modulators 220, for example, using Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK) modulation. The modulated CAZAC sequence is then transmitted after performing an Inverse Fast Fourier Transform (IFFT). Each RS is transmitted through the non-modulated CAZAC sequence after performing an IFFT 240.
FIG. 3 illustrates a transmitter block diagram for the PUCCH structure in FIG. 1.
Referring to FIG. 3, the HARQ-ACK information modulates a CAZAC sequence 310 which, without modulation, is also used for the RS. A controller 320 selects the first and second PRBs for transmission of the CAZAC sequence in the first and second slots of the PUCCH sub-frame and controls a sub-carrier mapper 330. The sub-carrier mapper 330 maps the first and second PRBs to the CAZAC sequence according to the control signal from the controller 320, respectively, an IFFT 340 performs IFFT, and a Cyclic Shift (CS) mapper 350 cyclically shifts the output of the IFFT 340. Finally, the CP inserter 360 inserts a CP to the signal output by the CS MAPPER 350, and a filter 370 performs time windowing to generate a transmitted signal 380. A UE is assumed to apply zero padding in REs that are not used for its signal transmission and in guard REs (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not shown.
FIG. 4 illustrates a receiver block diagram for the PUCCH structure in FIG. 1.
Referring to FIG. 4, an antenna receives an analog signal and after passing through further processing units, e.g., filters, amplifiers, frequency down-converters, and analog-to-digital converters (not shown) a digital received signal 410 is then filtered by a filter 420 and the CP is removed by a CP remover 430. Subsequently, the CS is restored by CS demapper 440, a Fast Fourier Transform (FFT) is applied by FFT 450, a controller 465 selects the first and second PRBs of the signal transmission in the first slot and second slots, respectively, and controls a sub-carrier demapper 460. The sub-carrier demapper 460 demaps the first and second PRBs according to the control signal from the controller 465, and the signal is correlated by multiplier 470 with a replica of the CAZAC sequence 480. The output 490 can then be passed to a channel estimation unit, such as a time-frequency interpolator, for an RS, or to a detection unit for the CAZAC sequence modulated by HARQ-ACK bits.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences and can be allocated to different UEs to achieve orthogonal multiplexing of HARQ-ACK signal transmissions in the same PRB. If Ts is the symbol duration, a number of such CSs is approximately └Ts/D┘, where D is the channel propagation delay spread, and └ ┘ is the floor function which rounds a number to its immediately lower integer.
In addition to orthogonal multiplexing of HARQ-ACK signals from different UEs in the same PRB using different CSs of a CAZAC sequence, orthogonal multiplexing can also be achieved in the time domain using Orthogonal Covering Codes (OCC).
For example, in FIG. 2, the HARQ-ACK signal can be modulated by a length-4 OCC, such as a Walsh-Hadamard (WH) OCC, while the RS can be modulated by a length-3 OCC, such as a DFT OCC (not shown). Accordingly, the multiplexing capacity is increased by a factor of 3 (determined by the OCC with the smaller length). The sets of WH OCCs, {w0, w1, w2, w3}, and DFT OCCs, {D0, D1, D2}, are respectively
            [                                                  W              0                                                                          W              1                                                                          W              2                                                                          W              3                                          ]        =          [                                    1                                1                                1                                1                                                1                                              -              1                                            1                                              -              1                                                            1                                1                                              -              1                                                          -              1                                                            1                                              -              1                                                          -              1                                            1                              ]                  and      ⁢                          [                                                  D              0                                                                          D              1                                                                          D              2                                          ]        =                  [                                            1                                      1                                      1                                                          1                                                      ⅇ                                                      -                    j2π                                    /                  3                                                                                    ⅇ                                                      -                    j4π                                    /                  3                                                                                        1                                                      ⅇ                                                      -                    j4π                                    /                  3                                                                                    ⅇ                                                      -                    j2π                                    /                  3                                                                    ]            .      
Table 1 presents a mapping for the PUCCH resource nPUCCH used for HARQ-ACK signal and RS transmission to an OCC noc and a CS α assuming 6 CS per symbol and a length-3 OCC. If all resources within a reference PUCCH PRB are used, the resources in the next PRB immediately following the reference PUCCH PRB can be used.
TABLE 1PUCCH Resource Mapping to OCC and CS.OCC noc for HARQ-ACK and for RSCS αW0, D0W1, D1W3, D20nPUCCH = 0nPUCCH = 61nPUCCH = 32nPUCCH = 1nPUCCH = 73nPUCCH = 44nPUCCH = 2nPUCCH = 85nPUCCH = 5
The SAs in the PDCCH are transmitted in elementary units that are referred to as Control Channel Elements (CCEs). Orthogonal Frequency Division Multiplexing (OFDM) is assumed as the DL transmission method. Each CCE includes a number of REs and the UEs are informed of the total number of CCEs, NCCE, in a DL sub-frame through the transmission of a Physical Control Format Indicator CHannel (PCFICH) by the NodeB. The PCFICH indicates the number of OFDM symbols used for the PDCCH transmission in the respective DL sub-frame.
For a Frequency Division Duplex (FDD) system, the UE determines nPUCCH as nPUCCH=nCCE+NPUCCH, where nCCE is the first CCE of the respective DL SA and NPUCCH is an offset configured by higher layers, such as a Radio Resource Control (RRC) layer, and can be informed to UEs through a DL broadcast channel.
A one-to-one mapping can exist between the PUCCH resources (PRB, CS, OCC) for HARQ-ACK signal transmission and the CCEs of the respective DL SA transmission. For example, if a single resource is used for HARQ-ACK signal transmission, the single resource may correspond to the CCE with the lowest index for the respective DL SA.
FIG. 5 illustrates a transmission of DL SAs using CCEs in respective PDCCHs.
Referring to FIG. 5, a DL SA for UE1 uses CCEs 501, 502, 503, and 504, a DL SA for UE2 uses CCEs 511 and 512, a DL SA for UE3 uses CCEs 521 and 522, and a DL SA for UE4 uses CCE 531. After cell-specific bit scrambling, modulation, and mapping to DL REs 540, each DL SA is transmitted in a PDCCH 550. Thereafter, UE1, UE2, UE3, and UE4 can use respectively nPUCCH=0, nPUCCH=4, nPUCCH=6, and nPUCCH=8 for their HARQ-ACK signal transmissions. Alternatively, if multiple CCEs are used to transmit a DL SA, HARQ-ACK information may be conveyed by the modulated HARQ-ACK signal and also by the selected PUCCH resource.
For a Time Division Duplex (TDD) system, multiple DL sub-frames may be linked to a single UL sub-frame in the sense that HARQ-ACK signal transmissions from UEs in response to DL SA receptions in these multiple DL sub-frames will occur in the same UL sub-frame. This set of DL sub-frames will be referred to as bundling window. To avoid having to always provision for the maximum PUCCH HARQ-ACK resources by always assuming the maximum PDCCH size in each DL sub-frame in the bundling window, the PUCCH resource indexing for HARQ-ACK signal transmission may exploit possible variations in the PDCCH size among DL sub-frames.
Denoting the number of DL sub-frames in the bundling window by M, the DL sub-frame index by m=0, 1, . . . , M−1, the number of CCEs for a PCFICH value of p(N0=0) by Np, and the first DL SA CCE in sub-frame m by nCCE (m), a PUCCH resource indexing for HARQ-ACK signal transmission can be as described below. The UE first selects a value pε{0, 1, 2, 3} providing Np≦nCCE(m)<Np+1 and uses nPUCCH=(M−m−1)×Np+m×Np+1+nCCE(m)+NPUCCH as the PUCCH resource for HARQ-ACK signal transmission in response to DL SA reception in DL sub-frame m, where Np=max{0,└[NRBDL×(NscRB×p−4)]/36┘}, NRBDL is the number of PRBs in the DL operating BW, and a CCE includes 36 REs.
The above indexing is based on interleaving the blocks of PUCCH resources for HARQ-ACK signal transmissions in an UL sub-frame that are linked to blocks of CCEs located in the first, second, or third PDCCH OFDM symbol in respective DL sub-frames. Interleaving, instead of serial concatenation of HARQ-ACK resources assuming the maximum PDCCH size in each DL sub-frame, allows for savings in the PUCCH resources for HARQ-ACK signal transmissions, when the PDCCH size in some DL sub-frames is not the maximum.
FIG. 6 illustrates block interleaving of PUCCH resources when there are 3 DL sub-frames in a bundling window.
Referring to FIG. 6, the PDCCH size is one OFDM symbol in the first DL sub-frame 610, three OFDM symbols in the second DL sub-frame 620, and two OFDM symbols in the third DL sub-frame 630. A total of 3N1 PUCCH resources 640 are first reserved for the first PDCCH OFDM symbol for each of the three DL sub-frames 640A, 640B, and 640C. Subsequently, a total of 2N2 PUCCH resources 650 are reserved for the second PDCCH OFDM symbol of the second 650B and third 650C DL sub-frames. Finally, N3 PUCCH resources 660 are reserved for the third PDCCH OFDM symbol of the second 660B DL sub-frame.
In order to increase the supportable data rates in a communication system, aggregation of multiple Component Carriers (CCs) is considered in both the DL and the UL to provide higher operating BWs. For example, to support communication over 60 MHz, Carrier Aggregation (CA) of three 20 MHz CCs can be used. A PDSCH reception in a DL CC is scheduled by a respective DL SA that is transmitted as illustrated in FIG. 5.
The transmission of HARQ-ACK signals associated with PDSCH receptions in multiple DL CCs can be in the PUCCH of a single UL CC, which will be referred to as a UL Primary CC (UL PCC) and can be UE-specific. Separate resources can be RRC configured in the UL PCC for HARQ-ACK signal transmissions in response to DL SA receptions in multiple DL CCs.
FIG. 7 is a diagram illustrating resource allocation in an UL CC for HARQ-ACK signal transmissions corresponding to DL SAs received in 3 DL CCs.
Referring to FIG. 7, the HARQ-ACK signal transmissions corresponding to PDSCH receptions in 3 DL CCs, DL CC1 710, DL CC2 720, and DL CC3 730, occur in the UL PCC 740. The resources for HARQ-ACK signal transmission corresponding to DL SA receptions in DL CC1, DL CC2, and DL CC3 are respectively in a first set 750, second set 760, and third set 770 of PUCCH resources.
If the provisioned PUCCH resources for HARQ-ACK signal transmissions consider the maximum number of PDCCH CCEs, the resulting overhead can be substantial. As a UE receiving PDCCH in a subset of DL CCs may not know the PDCCH size in other DL CCs, it may not know the number of respective PUCCH resources. Consequently, the maximum number of PUCCH resources, corresponding to the maximum number of PDCCH CCEs in each DL CC, is assumed. If less than the maximum of PUCCH resources are used in a sub-frame, the remaining PUCCH resources cannot typically be utilized for other transmissions, resulting in BW waste.
In addition to the PUCCH structure in FIG. 1, another PUCCH structure for HARQ-ACK signal transmission in response to DL SA receptions in multiple DL sub-frames (TDD) and/or in multiple DL CCs (CA) jointly codes the OHARQ-ACK HARQ-ACK information bits using, for example, a block code such as the (32, OHARQ-ACK) Reed-Mueller (RM) code.
FIG. 8 is a diagram illustrating a conventional PUCCH structure in one sub-frame slot using DFT Spread OFDM (DFT-S-OFDM) for the HARQ-ACK signal transmission.
Referring to FIG. 8, after encoding and modulation, using respectively, for example, a (32, OHARQ-ACK) RM block code punctured to a (24, OHARQ-ACK) RM code and QPSK modulation (not shown), a set of the same HARQ-ACK bits 810 is multiplied by a multiplier 820 with elements of an OCC 830 and is subsequently DFT precoded 840. For example, for 5 DFT-S-OFDM symbols per slot used for HARQ-ACK signal transmission, the OCC has a length of 5 and can be either {1, 1, 1, 1, 1}, or {1, exp(j2□/5), exp(j4□/5), exp(j6□/5), exp(j8□/5)}, or {1, exp(j4□/5), exp(j8□/5), exp(j2□/5), exp(j6□/5)}, or {1, exp(j6□/5), exp(j2□/5), exp(j8□/5), exp(j4□/5)}, or {1, exp(j8□/5), exp(j6□/5), exp(j4□/5), exp(j2□/5)}.
The output is passed through an IFFT 850 and is then mapped to a DFT-S-OFDM symbol 860. As the previous operations are linear, their relative order may be inter-changed. Because the HARQ-ACK signal transmission in the PUCCH is assumed to be in one PRB, which includes NscRB=12 REs, 24 encoded HARQ-ACK bits are transmitted in each slot with QPSK modulation (12 QPSK symbols). The same or different HARQ-ACK bits may be transmitted in the second slot of the sub-frame. RSs are also transmitted in each slot using a CAZAC sequence 870, as previously described, to enable coherent demodulation of the HARQ-ACK signals.
FIG. 9 illustrates a UE transmitter block diagram for the PUCCH structure in FIG. 8.
Referring to FIG. 9, HARQ-ACK bits 905 are encoded and modulated by an encoder/modulator 910 and then multiplied by multiplier 920 with an element of the OCC 925 for the respective DFT-S-OFDM symbol. After DFT precoding by DFT 930, controller 950 selects the REs of the assigned PUCCH PRB and the sub-carrier mapper 940 maps the REs according to the control signal from the controller 950. IFFT is performed by IFFT 960 and a CP inserter 970 and a filter 980 insert a CP and filter the transmitted signal 990, respectively.
FIG. 10 illustrates a NodeB receiver block diagram for the PUCCH structure in FIG. 8.
Referring to FIG. 10, after an antenna receives the analog signal and further processing, a digital signal 1010 is filtered by filter 1020 and the CP is removed by CP remover 1030. Subsequently, an FFT 1040 applies FFT, a controller 1055 selects the REs used by the UE transmitter and the sub-carrier demapper 1050 demaps the REs according to the control signal from the controller 1055. IDFT 1060 applies an IDFT, a multiplier 1070 multiples the output from the IDFT 1060 with an OCC element 1075 for the respective DFT-S-OFDM symbol, an adder 1080 sums the outputs for the DFT-S-OFDM symbols conveying HARQ-ACK signals over each slot, and a demodulator/decoder 1090 demodulates and decodes the summed HARQ-ACK signals over both sub-frame slots to obtain the transmitted HARQ-ACK bits 1095. Well known receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity.
Although the PUCCH structure illustrated in FIG. 8 can support HARQ-ACK payloads larger than a few bits, it still requires large PUCCH overhead as HARQ-ACK signal transmissions from at most 5 UEs (as determined by the OCC length) can be accommodated per PRB. Unlike the PUCCH structure illustrated in FIG. 1, the HARQ-ACK signal transmission resource for the PUCCH structure illustrated in FIG. 8 cannot be implicitly determined from PDCCH CCEs and is configured for each UE through RRC signaling. As most UEs do not usually have HARQ-ACK signal transmission in a sub-frame, if the provisioned PUCCH resources accommodate a unique resource for each UE, the resulting overhead can be substantial, as unused resources cannot typically be utilized for other transmissions, resulting in BW waste.
Instead of having separate HARQ-ACK resources for each UE, HARQ-ACK resource compression may be applied to reduce PUCCH overhead in a UL PCC. However, even though HARQ-ACK resource compression reduces the probability of resource waste, NodeB scheduler restrictions are required as collisions of HARQ-ACK resources should be avoided for UEs with shared HARQ-ACK resources.
Therefore, there is a need to reduce the PUCCH resources for HARQ-ACK signal transmissions in response to DL SAs received in multiple DL CCs or multiple DL sub-frames.
There is another need to avoid collisions for HARQ-ACK signal transmissions from multiple UEs that share the same set of PUCCH resources without imposing strict NodeB scheduler restrictions.
Finally, there is another need to determine rules for assigning PUCCH resources for HARQ-ACK signal transmissions from UEs.